1. Field of the Invention
The present invention relates to the fabrication of photomasks useful in the manufacture of integrated circuits.
2. Background of the Related Art
Photolithography techniques use light patterns and photoresist materials deposited on a substrate surface to develop precise patterns on the substrate surface prior to the etching process. In conventional photolithographic processes, a photoresist is applied on the layer to be etched, and the features to be etched in the layer, such as contacts, vias, or interconnects, are defined by exposing the photoresist to a pattern of light through a photolithographic photomask which corresponds to the desired configuration of features. A light source emitting ultraviolet (UV) light, for example, may be used to expose the photoresist to alter the composition of the photoresist. Generally, the exposed photoresist material is removed by a chemical process to expose the underlying substrate material. The exposed underlying substrate material is then etched to form the features in the substrate surface while the retained photoresist material remains as a protective coating for the unexposed underlying substrate material. Since photomasks are used repeatedly to create device patterns, quality control of photomask manufacturing is very important.
Photolithographic photomasks, or reticles, include binary (or conventional) photomasks and phase shift masks (PSM), which could be used in sub 0.13μm technology. Binary (or conventional) masks typically include a substrate made of an optically transparent silicon based material, such as quartz (i.e., silicon dioxide, SiO2), having an opaque light-shielding layer of metal, such as chromium, on the surface of the substrate. Phase shift masks improve the resolution of the aerial image by phase shifting. The principle of phase shift mask is described in P. 230-234 of Plummer, Deal and Griffin, “Silicon VLSI Technology Fundamentals, Practice and Modeling”, 2000 by Prentice Hall, Inc. Phase shift masks could be either attenuated phase shift or alternate phase shift mask. An attenuated phase shift mask typically includes a substrate made of an optically transparent silicon based material, such as quartz, having a translucent layer of material, such as molybdenum silicide (MoSi) or molybdenum silicon oxynitride (MoSiON), on top. When the photolithographic light, e.g. at 248 nm wavelength, shines through the patterned mask surface covered by the translucent layer, the transmission (e.g. 6% at 248 nm wavelength) and the thickness of the translucent layer create a phase shift, e.g., 180°, compared to the photolithographic light that shines through the patterned mask surface not covered by the translucent layer. An alternate phase shift mask typically includes a substrate made of an optically transparent silicon based material, such as quartz, which is etched to a certain depth to create a phase shift with the un-etched transparent substrate when the photolithographic light shines through the patterned mask. It also has a chrome layer with the same pattern as the quartz. There is another type of phase shift mask, the Chromeless Phase Lithography (CPL) Mask, which has the chrome layer removed.
Photomasks allow light to pass therethrough in a precise pattern onto the substrate surface. The metal layer on the photomask substrate is patterned to correspond to the features to be transferred to the substrate. The patterns on the photomask could be 1×, 2× or 4× the size of patterns that will be patterned on the wafer substrate. Typically, a photolithographic stepper reduces the image of the photomask by 4× and prints the pattern on the photoresist covering the wafer surface. Conventional photomasks are fabricated by first depositing one to two thin layers of metal, which could either be opaque or translucent depending on the types of masks being formed, on a substrate comprising an optically transparent silicon based material, such as quartz, and depositing a photoresist layer on substrate. The photomask is then patterned using conventional laser or electron beam patterning equipment to define the critical dimensions in the photoresist. The top metal layer, typically opaque, is then etched to remove the metal material not protected by the patterned photoresist, thereby exposing the underlying silicon based material. For a binary mask, the photomask is formed after the metal etching step. While for attenuate and alternate phase shift masks, additional photoresist patterning and etching of transparent substrate or translucent metal layer are needed to form the photomask.
Since photomasks are used repeatedly to create device patterns, the accuracy and tight distribution of the critical dimensions, and the phase shift angle and its uniformity across the substrate are key requirements for binary and phase shift photomasks. For alternate phase shift mask, the phase angle is affected by the depth of the transparent material, such as quartz. Since precise control of the phase shift is very important, the etching of the transparent material, such as quartz, is often accomplished after multiple etching processes and multiple etch depth measurements to ensure phase shift of the mask is within control limit. If the etch depth measurement is performed in a system not integrated with the etching system, process cycle time could be very long and the approach could increase the total defect counts.
Therefore, there remains a need in the art for an integrated metrology tool to measure etch depth (or phase shift angle) of photomask in a semiconductor photomask processing system.